Electrical insulator



Oct. 15, 1940. D H, mm ET A 2,218,497

ELECTRICAL INSULATOR Filed Dec. 5, 1958 2 SheetsShee+. l

INVENTORS D H. 5M ITH H. EL

H WEE ER ATTORNEY 1940- D. H. SMITH ET AL ELECTRICAL INSULATOR FiledDec. 5, 1938 2 Sheets-Sheet FIG. 6

FIG. 5

INVENTORS D.H.SM 1TH BY HH. WH EILER ATTORNEY Patented Oct. 15, 1940UNITED STATES PATENT OFFICE ELECTRICAL INSULATOR Application December 3,1938, Serial No. 243,762

7 Claims.

This invention relates generally to electrical insulators, and moreparticularly to pole line insulators for use with telegraph, telephoneand other communication circuits.

- The signaling currents employed in communication circuits are usuallyof small amplitude, and

a definite and considerable portion of each signal impulse transmittedis required for operating the receiving apparatus, such as relays intelegraph l0 circuits. While repeaters are employed at convenientlocations, in certain instances the re- .peater points are necessarilyspaced considerable distances apart and the signals reaching thereceiving apparatus may become considerably attenuated, and since themargin of operation on communication circuits under favorable conditionsis relatively small, any substantial loss of signaling current due toline leakage seriously interferes with the normal operation of thesystem.

Also, communication circuits frequently are disposed along highways andrailroad right of ways in connection with other signaling and powerlines,-and are subject to interference therefrom,

and a considerable portion of the signaling cur rent is required tooverride the transient or interfering currents set up therein because ofsuch adjacent signaling and power lines, and this further reduces theoperating margin of the com munication signals.

In order to maintain the insulation of communication circuits as high aspossible, it has heretofore been the practice to employ vitreousmaterial, such as glass or porcelain, for pole line insulators, sincesuch material, especially glass,

was found the most practicable for such purposes. Glass and othervitreous insulators, however, are subject to various disadvantages. Poleline insulators form ideal targets for malicious persons who throwstones and missiles or shoot at them,

with resultant breakage of the insulators. When an insulator is thuscracked or broken, it enables dirt and other foreign matter carried bythe elements to collect in the cracked or broken portion and this formsa conducting leakage path over the surface of the insulator,particularly in rainy or humid weather, and if a portion of the skirt orpetticoat of the insulator is broken away, this reduces the length ofthe leakage path over the insulator with the result that the insulatorloses its desired insulation value. Also, when insulators are cracked orbroken, there is a tendency for the insulators to be pulled or to flyoff the pins on which they are supported, and the wire carried by theloose insulator may ground on the 5 crossarm or swing into the adjacentwire, thus either grounding or short-circuiting the communicationcircuit or causing the same to be crossed with an adjacent circuit.

In an effort to avoid breakage of insulators, it has heretofore beenproposed to employ shields, usually of metal, to protect the insulator.The installation and maintenance of such shields, however, is expensiveand is disadvantageous because the shields reduce the clearance betweenadjacent wires and also introduce additional uno desired capacitybetween the conductors of a circuit and between these conductors andground, which capacity effect materially attenuates the signals.

A further disadvantage of glass and other 15 vitreous insulators is thatwhen dry and clean they may provide a negligible leakage path for thesignal current, but when exposed to rain or moisture they are subject toconsiderable surface leakage, and thus the insulation of the cir- 20cuit varies between wide limits and may necessitate frequentreadjustment of the terminal and repeater apparatus, or the insulationof the circuit may become so low as to render the circuit inoperative.The most severe conditions of sur- 215 face leakage in such insulatorsoccur during periods of rain, fog, and high humidity. During rainfallthe outer surface of the insulator becomes completely wet and the innersurface also becomes wet due to splash and condensation. By reason ofthe relatively low interfacial tension characteristics of the glass withwater, the splash or condensation on the inner surface of the insulatorusually produces a continuous film of water on the surface, therebymaterially increasing leakage over the insulator. During periods of fogthe particles of moisture deposited on the insulator surfaces also tendto run together and form a continuous film of water. During periods ofhigh humidity and varying temperature con- 40 ditions, the moisturecondenses on the surfaces of the insulator, and this also forms acontinuous film of water. Even though the temperature conditions duringperiods of high humidity are not such as to induce condensation,nevertheless con 45 siderable leakage is often experienced as a resultof the absorption of moisture from the air by the hygroscopic freealkalies and other chemicals which are present in the surface of theglass or other vitreous material.

In an effort to prevent the formation of continuous films of water,particularly on the inner surfaces of the insulator during periods ofrain, fog and condensation, it has heretofore been proposed to employ aspecial iridescent coating on 55 cult to o l glass insulators in themanner disclosed in the Wheeler and McGinnis application, Ser. No.621,478, filed July 8, 1932, now Patent No. 2,165,773, issued July 11,1939, which coating exhibits the characteristic of high interfacialtension with water. Such insulators, however, retain various inherentdisadvantages of glass and other vitreous insulators, in that they arerelatively fragile subject to breakage, and have to be made large andbulky to provide a reasonable measure of strength. Furthermore, thematerial of such insulators has a relatively high coefficient of thermalconductivity. This results in a relatively large amount of condensationof moisture on both the outer and inner surfaces of the insulatorsinsulators are subjected to currents moisture-bearing air. Since theseinsulators are large and bulky, the dielectri losses are considerable,and it is diffiin the close spacing which is desirable between onductorsin high frequency communication circuits to counteract interference andcrosstalk.

Various substitutes, including hard vulcanized rubber, have beensuggested in place of glass and porcelain for insulators, but none ofthese substitutes has proved electrically and mechanically satisfactoryin service, and in none of these ill-- sulators has the surface portionthereof that comprises a leakage path over the insulator had the desiredhigh interfacial tension with water.

The object of this invention is an insulator in which the foregoingdisadvantages of vitreous insulators and substitutes therefor areobviated.

A specific object is an insulator substantially composed of a rubbercompound or the like, which has a surface that exhibits high interfacialtension with water.

Anotler object is an insulator of the character disclosed, whichexhibits and retains in service high nsulation characteristics.

L rurther object is an insulator of the character disclosed, in whichthe dielectric losses are low.

Still another object is an insulator which exhibits the desired physicaland electrical properties, and which facilitates close spacing betweenadjacent wires of a high frequency circuit.

Other objects and attendant advantages will appear from the followingdetailed description taken in connection with the accompanying drawingsin which:

Fig. is a view, in elevation, of a communication type insulatorsubstantially composed of a rubber compound in accordance with myinvention;

Fig. 2 is a longitudinal section of Fig. 1, showing the manner in whichthe insulator is mounted on its supporting pin;

Fig. 3 illustrates the relative sizes and configurations of an insulatorembodying my invention and a standard glass insulator of the typeheretofore employed for the same type of service;

Fig. 4 shows a modified form of an insulator in accordance with theinvention;

Fig. 5 illustrates the small spacing and clearance present with standardglass insulators when it is attempted to obtain the optimum spacingbetween adjacent conductors in high frequency communication circuits;and

Fig. 6 illustrates the large effective clearance obtainable between theconductors of the circuit of Fig. 5, when supported by insulators inaccordance with the present invention.

Referring particularly to Figs. 1 and 2, there is shown a. communicationtype insulator substantially composed of a vulcanized rubber compound inaccordance with the invention, the insulator having a crown portion [0,a wire groove portion II for receiving the line wire, a reentrant skirtor petticoat portion l2, and a pin hole I3 for receiving the threadedportion Ha of a. metal in sulator pin M which is secured, by a reducedportion Mb thereof, to the crossarm of a pole line in known manner.

The losses in signaling current occasioned by line insulators are, ingeneral, due to three causes: First, the leakage from the lineconductor, including its tie wire, over the outer surface of theinsulator and under its skirt or petticoat to the supporting insulatorpin; second, the leakage from the line conductor and tie wire throughthe material of the insulator to the insulator pin; and third, thedielectric losses. As hereinbefore stated, the first cause of losses insignaling current, which is surface leakage, causes considerable troubleunder rainy or humid conditions in insulators of the types heretoforeemployed and. in which both the outer and inner surfaces of theinsulator may become covered with a film of moisture. The second causeof losses will also become considerable if the material of the insulatordoes not have a high specific volumetric resistance and maintain thishigh resistance in service. With respect to the third cause of losses,which are dielectric losses, these may be considerable if the materialof the insulator has a high dielectric constant or if the mass of theinsulator material disposed in the static held about the line wire andthe tie wire is large, these losses becoming increasingly important inhigh frequency circuits since they increase with the frequency of thecommunication currents employed.

In accordance with the preferred embodiment surfaces to give the effectobtained by the lridized coating in the aforesaid Wheeler and McGinnispatent. In other words, the moisture occurring on the surface of theinsulator by reason of any condensation, splash or direct rain, forms inseparate globules which run off the surface freely without leaving atrail of free moisture behind, and thus there is produced a nonwettingsurface which maintains high insulation values under the most adverseweather conditions, thereby substantially reducing any loss of signalingcurrent due to line leakage and obviating the necessity of frequentreadjustment of the repeater and receiving apparatus employed intelegraph and other communication circuits.

Among the substances which when compounded with rubber have been foundto produce a. surface having the desired high interfacial tension withwater, are hydrocarbon waxes, including paraffin, ceresin, ozokerite,and amorphous types such as wax tailings, mineral beeswax," and thelike, vegetable waxes such as carnauba, montan, and others, and animalwaxes such as stearin, stearic acid, beeswax, Chinese insect wax,tallow, spermaceti, and the like. Also, synthetic waxes, such ashydrogenated montan wax, and

higher alcohols, and substances chemically similar to waxes, such aslanolin, may be used. The proportions of such substances may vary withinrelatively wide limits; satisfactory results have been obtained by usingone to five percent by weight of various of the foregoing. substances.These substances may be used in lesser or greater proportions, withvarying results, except that a sufficient amount of the substance mustbe employed to cause the surface of the rubber compound to have thedesired high interracial tension with water, without employing suchamount of the substance as deleteriously affects the physical propertiesof the rubber compound. The desired proportions of these substancesrelative to the rubber may readily be determined empirically in eachinstance. It will be apparent that various waxes, synthetic waxes andsubstances chemically similar to waxes, other than those specificallymentioned, and derivatives thereof. may be employed. In thespecification and claims, wherever applicable, the term waxlike" isemployed in a generic sense to define waxes, synthetic waxes, substanceschemically similar to waxes, wax substitutes and/or equivalents thereof,which exhibit high interfacial tension with water and/or when compoundedwith or incorporated in the rubber compound cause the surface of theinsulator to have the desired high interfacial tension with water.

As is well known, rubber compounds ordinarily include numerousingredients other than rubber, such as ingredients for reinforcing,filling, coloring, softening, vulcanizing, and as age-resistors, some ofthe well known compounding ingredients including silica, clay, carbon,zinc oxide, sulphur and magnesium carbonate. These compoundingingredients, however, should be of such character or employed in suchamounts that they will not, to a material extent, detrimentally affectthe desired physical and electrical characteristics of the rubbercompound, particularly in regard to its specific electrical resistance,dielectric losses and the like. Also, various synthetic and artificialrubber and rubber-like products maybe used in the compound incombination with or to replace the rubber in whole or in part, thepermissible extent of such addition or replacement being limited only inso far as such substitutes undesirably affect the desired physical andelectrical properties of the insulator. In the specification and claims,wherever applicable, the term rubber compound is employed in a genericsense to define compounds of rubber and/or synthetic and artificialrubber, and rubher substitutes and rubber-like products, including thevarious other compounding ingredients and the waxlike substance orsubstances employed, and the term vulcanized is employed in a genericsense to define any process or curing treatment of the foregoing rubbercompounds to cause them to have and maintain in service the necessaryphysical characteristics to render the same suitable for use as aninsulator of the character disclosed. By varying the amounts andcharacter of the compouding ingredients, includ ing the vulcanizingagents, the desired degree of softness or hardness and other physicalcharacteristics of the rubber compound can be obtained and, in so far asobtaining the high interfacial tension with water, the rubber compoundmay be relatively hard when vulcanized, although, as above stated, thesoft vulcanized rubber compound is preferable.

In addition to the advantages attendant to the high interfacial tensionwith water, it has been found that an insulator composed of a softvulcanized rubber compound in accordance with the invention isnon-porous and is substantially non-absorbent, and even though certainof the materials used as fillers, pigments and other compoundingingredients may in themselves be hygroscopic, such materials aresubstantially sealed in the surface of the insulator against moisture bythe rubber and the substance or substances employed for causing thesurface to have a high interfacial tension with water. Thus theformation of conducting fllms resulting from the presence of freealk-alies and other chemicals in glass and vitreous materials areobviated.

Furthermore, an insulator composed of a soft vulcanized rubber compoundin accordance with the invention has a much lower coefiicient of thermalconductivity than glass, hard rubber and other materials heretoforeproposed for line insulator purposes. For example, the composition ofthe insulator herein disclosed may be such that it has a coefficient ofapproximately ther- 7 mal conductivity of .109, whereas the coeflicientof glass varies between approximately .33 and .5, and that of hardrubber is approximately. .9, these values being expressed in the Englishsystem of measures, i. e., B. t. .u. per hour per square foot per footthickness per degree of Fahrenheit difference in temperature. It will beappreciated thathard rubbers of specifically different formulas willpresent a considerable variation in values of thermal conductivity; thisis also true with regard to the various glasses, and to a lesser extentto soft rubbers. For example, depending upon the particular formulasemployed, hard rubber may have a coefficient of thermal conductivityranging down to .093 B. t. u.; the coefficient of Pyrex glass may beapproximately .63 B. t. u.; and soft rubber .075 to .093 B. t. u.; theimportant consideration, however, is that of the relative values of softvulcanized rubber and glass. Compounds of such rubbers having compoundedtherewith a. waxlike substance or substances in accordance with thepresent invention will have substantially the same coefficients ofthermal conductivity as those of the rubbers themselves. By reason ofthe low coeflicient of thermal conductivity obtainable with theinsulator disclosed herein, the surface of the insulator, underconditions of varying temperature, assumes more rapidly the temperatureof the surrounding air, and this minimizes the amount of condensation ofmoisture. Furthermore, the skirt in the insulator of this invention maybe made relatively thin, and as a result the time required to bring thematerial of this portion of the insulator to the temperature of thesurrounding air is short, and hence the amount of condensation isfurther reduced.

Because of the foregoing characteristics, insulators in accordance withthe invention retain the desired insulating characteristics for aconsiderably longer period of time than vitreous and other insulatorsheretofore known. Tests have shown that insulators embodying ourinvention which have been subjected to severe conditions of rain, fogand dirt have retained their high insulating properties to a degreesubstantially in'excess of that exhibited by vitreous insulators,including iridized insulators, exposed to the same conditions.

Another important advantage of the use of a soft vulcanized rubbercompound in accordance with our invention is that the insulator isnon-breakable. Also, as disclosed in Fig. 2, the pin hole l3 in theinsulator does not have to be threaded for engagement with the threadedportion of the pin it, but may comprise a smooth bore since the diameterof the pin hole may be made slightly less than the outer diameter of thepin, the elasticity of the rubber compound enabling the insulatorreadily to be forced on the pin, after which it resiliently grips thepin and thus minimizes or precludes the possibility of the insulatorunscrewing off the pin because of vibration or flying off the pinbecause of a pull exerted thereon by the line wire. Furthermore, theinsulator requires no wooden cob, shim or other cushioning materialbetween the metal pin and insulator which generally has to be employedin the case of vitreous insulators to prevent cracking of the insulatorsdue to the different temperature coefficients of expansion of theinsulator and pin. Since no cob or shim is required, the mass ofmaterial in the static field is greatly reduced, with consequentreduction of dielectric losses, and the dielectric losses present inwooden cobs are eliminated. It is, in fact, not necessary that theinsulator pin be provided with a threaded portion, since anyprotuberance or roughened surface will enable the insulator toeffectively grip the pin, and by reason of the elasticity of thematerial the insulator is adapted to receive pins with differentdiameters and different threads. If desired, and as shown in Fig. 4, theinsulator may be molded and vulcanized on the pin l5, and thus the pinand insulator become an integral structure so that there is nopossibility of the insulator flying off the pin. An additional advantageof the insulator is that it has a dampening effect on vibration set upin the line wires, and there is no injurious or abrasive effect on theline wires by reason of the wires rubbing against the insulator, as isthe case when vitreous insulators are employed.

Referring again to Fig. 2, it will be seen that the crown and wiregroove portions l0 and H are built up so that they are relativelyinflexible and thus enable the insulator to be rigidly mounted on aninsulator pin or other supporting member, and thereby preventdisplacement of the insulator on its support and accurately maintain thedesired clearance between the wire and the adjacent conductors and thesupporting crossarm, and rigidly support the line wire in apredetermined fixed position. The crown and wire groove portions,however, are sufiiciently elastic to cause them to withstand the forceof impact when struck by an object without permanent deformation of orinjury to said portions. As shown, the underlip Ila of the wire grooveis also relatively rigid and inflexible, so that it can hold the weightof the wire even when loaded with ice, at which time there may be aweight of from 300 to 400 pounds per span exerted on the pin by the linewire.

The skirt l2, however, may be provided with a very thin wall, thereby toobtain the benefits with respect to reduced condensation of moistureabove referred to, and to cause the skirt to be sufficiently flexible towithstand better and partially absorb the force of impact when struck byan object, the skirt thus materially preventing the force of impact frombeing communicated to the upper portion of the insulator and possi-- blycausing displacement of the insulator.

Fig. 3 illustrates the relative proportions and configurations of aninsulator in accordance with the present invention, shown in full linein the figure, and a conventional glass insulator i, the relative sizeand configuration of which is shown approximately by broken lines in thefigure. The two insulators are each designed for the same circuit or thesame class of service. In the insulator in accordance with the presentinvention, the largest diameter of the insulator, which is'across theouter edges of the reinforced lip portion Ila, is 1% inches whereas thelargest diameter of the glass insulator i, which is across the outersurfaces of the outer petticoat shown, is 3% inches, the necessaryoverall dimension of this insulator being approximately twice that ofthe insulator of our invention. Also, the overall height of theinsulator of the invention, from the top of the crown to the bottom ofthe skirt is 3 inches, whereas the overall height of the glass insulatori, from the top of its crown to the bottom of its drip point portion, isnearly 5 inches. Furthermore, the insulator of the invention requiresbut one skirt or petticoat l2 which may be made quite thin; a skirtwhich at its bottom portion l2a is only inch thick has been foundsatisfactory. Since no cob or shim is required, the inner diameter ofthe skirt may be appreciably reduced, so that the skirt may closelyapproach the pin on which the insulator is mounted, as will be seen fromFig. 2, and thereby appreciably reduce the splash effect and also reducethe amount of dirt and other foreign material that may come in contactwith the inner surface of the insulator. An insulator, such as shown inFig. 3, in which the inner diameter of the bottom portion 12a of theskirt is only 1% inches has been found suitable.

In the case of high frequency circuits it is often desirable to reducethe spacing between conductors of a circuit to limit cross-talk andinterference, as shown in Figs. 5 and 6. In Fig. 5 in which standardglass insulators are used, it will be seen that, with a desired closespacing between conductors, only a very limited separation is obtainablebetween the tie wire t of conductor w and the tie wire t' of conductorw, and that the separation between the insulators is still more limited.This condition not only results in undesirable increase in capacitybetween the conductors, but also results in the possibility of seriousleakage between the conductors due to spider webs and other foreignsubstances extending or lodging between the insulators and tie wires.With the same spacing between the conductors w and w of Fig. 6, in whichinsulators of the invention are employed, it will be seen that nearlytwice the spacing is obtained between the tie wires t and t, and thatthe spacing between the insulators is many times that obtainable by thestandard glass insulators, thereby preventing undesirable increase incapacity between the conductors, and reducing the likelihood of seriousleakage between the conductors of the circuit.

The invention disclosed herein which is not recited in the appendedclaims, particularly in regard to the mechanical features of aninsulator composed of a rubber compound, is not disclaimed but comprisesthe subject matter of a copending continuation-impart application of D.H. Smith and H. H. Wheeler, Serial No. 348,- 708, filed July 31, 1940.

While there are shown and described herein certain preferred substancesand embodiments, many other and varied forms, uses and substances willsugiest themselves to those versed in the art without departing from theinvention, and the invention is therefore not limited except as,indicated by the scope of the appended claims.

We claim:

1. An insulator for supporting a line conductor, adapted to be mountedon an insulator supporting member and to maintain high line insulationvalues over long periods of time and under adverse weather conditions,the body of said insulator being substantially composed of a vulcanizedrubber compound and having means for securing the same to said insulatorsupporting member, a wire groove portion for supporting said lineconductor in predetermined fixed position with respect to saidsupporting member, said vulcanized rubber compound comprising means forcausing the surface of the insulator body which forms a leakage pathover the insulator between said line conductor and insulator supportingmember to have and retain in service non-wetting characteristics, saidlast named means comprising a waxlike substance compounded with andforming a component of said vulcanized rubber compound and having theproperty of causing said surface to exhibit high interfacial tensionwith water, said waxlike substance being present in an amount sufllcientto produce the desired non-wetting surface charac teristics, but less inamount than that which to a substantial extent would detrimentallyaffect the physical characteristics of the vulcanized rubber compoundwhen thus employed as an insulator body.

2. An insulator for supporting a line conductor, adapted to be mountedon an insulator supporting member and to maintain high insulation valuesover long periods of time and under adverse weather conditions, the bodyof said insulator being substantially composed of a vulcanized rubbercompound and having means for securing the same to said insulatorsupporting member, a wire groove portion for supporting said lineconductor in predetermined fixed position with respect to saidsupporting member and a reentrant skirt portion for increasing theleakage path between the line conductor and insulator supporting member,said vulcanized rubber compound comprising means for causing the surfaceof the insulator body which forms a leakage path over the insulatorbetween said line conductor and insulator supporting member to have andretain in service non-wetting characteristics, said last named meanscomprising a waxlike substance compounded with and forming a componentof said vulcanized rubber compound and having the property of causingsaid surface to exhibit high interfacial tension with water, saidwaxlike substance being present in an amount suificient to produce thedesired non-wetting surface characteristics, but less in amount thanthat which to a substantial extent would detrimentally affect thephysical characteristics of the vulcanized rubber compound when thusemployed as an insulator body.

3. An insulator for supporting a line conductor, adapted to be mountedon an insulator supporting member and to maintain high line insulationvalues over long periods of time and under adverse weather conditions,the body of said insulator being substantiallycomposed of a vulcanizedrubber compound and having means for securing the same to said insulatorsupporting member, a wire groove portion for supporting said lineconductor in predetermined fixed position with respect to saidsupporting member and a reentrant skirt portion for increasing tl'leakage path between the line conductor and insulator supporting member,said skirt portion being relatively thin with respect to the skirtportion of an insulator composed ofvglass, porcelain and the like, saidvulcanized rubber compound comprising means for causing the surface ofthe insulator body which forms a leakage path over the insulator betweensaid line conductor and insulator supporting member to have and retainin service non-wetting characteristics, said last named means comprisinga waxlike substance compounded with and forming a component of saidvulcanized rubber compound and having the property of causing saidsurface to exhibit high interfacial tension with water, said waxlikesubstance being present in an amount suiilcient to produce the desirednon-wetting surface characteristics, but less in amount than that whichto a substantial extent would detrimentally afiect the physicalcharacteristics of the vulcanized rubber compound when thus employed asan insulator body.

4. A pole line insulator for supporting a line conductor, adapted to bemounted on an insulator supporting member and to maintain high lineinsulation values over long periods of time and under adverse weatherconditions, the body of said insulator being substantially composed of avulcanized rubber compound and having means for securing the same tosaid insulator supporting member, a wire groove portion for supportingsaid line conductor in predetermined fixed position with respect to saidsupporting member and a skirt portion for increasing the leakage pathbetween the line conductor and insulator supporting member, saidvulcanized rubber compound having a coeflicient of thermal conductivitysubstantially lower than that of glass and comprising means for causingthe surface of the insulator body which forms a leakage path over theinsulator between said line conductor and insulator supporting member,to have and retain in service non-wetting characteristics, said lastnamed means comprising a waxlike substance compounded withand forming acomponent of said vulcanized rubber compound and having the property ofcausing said surface to exhibit high interfacial tension with water,said waxlike substance being present in an amount sufficient to producethe desired non-wetting surface characteristics, but less in amount thanthat which to a substantial extent would detrimentally afi'ect thephysical characteristics of the vulcanized rubber compound when thusemployed as an insulator body.

5. An insulator for supporting a line conduc tor, adapted to be mountedon an insulator supporting member and to maintain high line insulationvalues over long periods of time and under adverse weather conditions,the body of said insulator being substantially composed of a softvulcanized rubber compound and having means for securing the same tosaid insulator supporting member, a wire groove portion for supportingthe line conductor in predetermined fixed position with respect to saidsupporting member, said soft vulcanized rubber compound having acoefficient of thermal conductivity substantially lower than that ofglass and comprising means for causing the surface of the insulator bodywhich forms a leakage path over the insulator between said lineconductor and insulator supporting member to have and retain in servicenon-wetting characteristics, said last named means comprising a waxlikesubstance compounded with and forming a component of said softvulcanized rubber compound and having the property of causing saidsurface to exhibit high interfacial tension with water, said waxlikesubstance being present in an amount sufficient to produce the desirednon-wetting surface characteristics, but less in amount than that whichto a substantial extent would detrimentally affect the physicalcharacteristics of the soft vulcanized rubber compound when thusemployed as an insulator body.

6. An insulator for supporting a line conductor, adapted to be mountedon an insulator supporting member and to maintain high line insulationvalues over long periods of time and under adverse weather conditions,the body of the insulator being substantially composed of a softvulcanized rubber compound which includes fillers, pigments and othercompounding ingredients for giving desired physical characteristics tothe compound, the character of said fillers, pigments and othercompounding ingredients and the relative proportions thereof being suchthat the compound has a high specific electrical resis tance, saidinsulator body having means for securing the same to said insulatorsupporting member, a wire groove portion for supporting said lineconductor in predetermined fixed position with respect to saidsupporting member, said soft vulcanized rubber compound comprising meansfor causing the surface of the insulator body which forms a leakage pathover the insulator between said line conductor and insulator supportingmember to have and retain in service non-wetting characteristics, saidlast named means comprising a waxlike substance compounded with andforming a component of said soft vulcanized rubber compound and havingthe property of causing said surface to exhibit high interfacial tensionwith water, said waxlike substance being present in an amount suflicientto cause said compound to have low hygroscopicity and also to producethe desired non-wetting surface characteristics, but less in amount thanthat which to a substantial extent would detrimentally affect thephysical characteristics of the rubber compound thus employed as aninsulator body.

7. A pole line insulator for supporting a line conductor, adapted to bemounted on an insulator supporting member and to maintain high lineinsulation values over long periods of time and under adverse weatherconditions, the body of the insulator being substantially composed of asoft vulcanized rubber compound which includes rubber and fillers,pigments and other compounding ingredients for giving desired physicalcharacteristics to the compound, the character of said fillers, pigmentsand other compounding ingredients and the proportions thereof relativeto the rubber being such that the compound has a high specificelectrical resistance, said insulator body .having means for securingthe same to said insulator supporting member, a wire groove portion forsupporting said line conductor in predetermined fixed position withrespect to said supporting member and a reentrant skirt portion [orincreasing the leakage path between the line conductor and insulatorsupporting member, said soft vulcanized rubber compound comprising meansfor causing the surface of the insulator body which forms a leakage pathover the insulator between said line conductor and insulator supportingmember to have and retain in service non-wetting characteristics, saidlast named means comprising a waxlike substance compounded with andforming a component of said soft vulcanized rubber compound and havingthe property of causing said surface to exhibit high interfacial tensionwith water, said waxlike substance being present in an amount suflicientto cause said compound to have low hygroscopicity and also to producethe desired non-wetting surface characteristics, but less in amount thanthat which to a substantial extent would detrimentally affect thephysical characteristics of the rubber compound thus employed as aninsulator body.

DONALD H. SMITH. HERBERT H. WHEELER.

